Starts an RMA access
epoch. Locks ensure that only the windows created by specific processes
can be accessed by those processes (and by no other processes) during that
epoch.

Locks are used to protect accesses to the locked target window effected
by RMA calls issued between the lock and unlock call, and to protect local
load/store accesses to a locked local window executed between the lock
and unlock call. Accesses that are protected by an exclusive lock will
not be concurrent at the window site with other accesses to the same window
that are lock protected. Accesses that are protected by a shared lock will
not be concurrent at the window site with accesses protected by an exclusive
lock to the same window.

The assert argument is used to provide assertions
on the context of the call that may be used for various optimizations. (See
Section 6.4.4 of the MPI-2 Standard.) A value of assert = 0 is always valid.

In a client/server environment in which clients connect to a server
and create windows that span both the client and the server, if a client
or server that has obtained a lock on such a window and then terminates
abnormally, the server or other clients may hang in a MPI_Win_lock call,
failing to notice that the peer MPI job has terminated.

Almost all
MPI routines return an error value; C routines as the value of the function
and Fortran routines in the last argument. C++ functions do not return errors.
If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then
on error the C++ exception mechanism will be used to throw an MPI:Exception
object.

Before the error value is returned, the current MPI error handler
is called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler;
the predefined error handler MPI_ERRORS_RETURN may be used to cause error
values to be returned. Note that MPI does not guarantee that an MPI program
can continue past an error.